Common Mechanisms of Drug Interactions with Sodium and T-Type Calcium Channels

Chris Bladen,Gerald W. Zamponi

doi:10.1124/mol.112.079715

Abstract

Voltage-gated sodium (Na(v)) and calcium (Ca(v)) channels play important roles in physiological processes, including neuronal and cardiac pacemaker activity, vascular smooth muscle contraction, and nociception. They are thought to share a common ancestry, and, in particular, T-type calcium (T-type) channels share structural similarities with Na(v) channels, both with regard to membrane topology and with regard to gating kinetics, including rapid inactivation. We thus reasoned that certain drugs acting on Na(v) channels may also modulate the activities of T-type channels. Here we show that the specific Na(v)1.8 blocker 5-(4-chlorophenyl-N-(3,5-dimethoxyphenyl)furan-2-carboxamide (A803467) tonically blocks T-type channels in the low micromolar range. Similarly to Na(v)1.8, this compound causes a significant hyperpolarizing shift in the voltage dependence of inactivation and seems to promote a slow inactivation-like phenotype. We further hypothesized that the structural similarity between T-type and Na(v) channels may extend to structurally similar drug-binding sites. Sequence alignment revealed several highly conserved regions between T-type and Na(v) channels that corresponded to drug-binding sites known to alter voltage-dependent gating kinetics. Mutation of amino acid residues in this regions within human Ca(v)3.2 T-type channels altered A803467 blocking affinity severalfold, suggesting that these sites may be exploited for the design of mixed T-type and Na(v) channel blockers that could potentially act synergistically to normalize aberrant neuronal activity.

Highlights

voltage-gated sodium channel (Nav) channels mediate the induction and propagation of action potentials in most electrically excitable cells (Yu and Catterall, 2004)
Given that T-type channels share structural similarities with Nav channels, we examined whether this compound may affect T-type channels at both therapeutic plasma and brain tissue concentrations (10–17 and 3–5 ␮M, respectively) (Jarvis et al, 2007)
We demonstrate that A803467 blocks T-type channels with high affinity, with IC50 values that fall into the range of therapeutic concentrations and that block of the Cav3.2 channel subtype seems to stabilize slow inactivation

Summary

Introduction

Nav channels mediate the induction and propagation of action potentials in most electrically excitable cells (Yu and Catterall, 2004). The mammalian genome encodes nine different types of Nav ␣ subunits that are functionally classified as either tetrodotoxin (TTX)-sensitive or TTX-resistant, with the latter exhibiting slower inactivation kinetics than other Nav channel subtypes (Waxman et al, 1999; Blair and Bean, 2002). The various Nav channel ␣ subunits share a common transmembrane topology of four homologous domains that each contain six membrane-spanning helices plus a p-loop. Whereas the ␣ subunits define the Nav channel isoform and contain all of the machinery to form a sodium-selective voltage activated channel, their functional properties are modulated by association with ancillary ␤1 and ␤2 subunits (for review, see Isom, 2001). G.W.Z. is a Canada Research Chair and an Alberta Innovates–Health Solutions Scientist. Chen Fong studentship and an Alberta Innovates–Health Solutions studentship award

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